Easiest is new engine, but, take the head off anyway for your peace of mind, and look for "tracking"... You will notice which cylinder is pushing into the coolant... Or even remove all the spark plugs (keep them in order), at least one will be different !
TIP: ALWAYS use a BMW gasket when you rebuild it !
I would personally sort your engine as apposed to a new unit !
DAZ

My M52 unit started to overheat, so I heard of K-Seal and launched a bottle full into the coolant... All fixed and runs sweet LOL

i couldnt find k seal anywhere so i tried super seal its a form of liquid glass
it seems to have worked no pressure building up i did remove the thermostat
so the liquid would flow better should i flush out the system now and refill with coolant thanx again...